1. Field of the Invention
The present invention relates to a light-emitting diode (LED) print head, and more particularly to an LED printer print head that uses an array of LEDs to produce the print images.
2. Description of the Prior Art
The increasing use of information processing devices such as word processors and the like has resulted in the emergence of various types of printers to output the information. For applications in which the emphasis is on quietness, high speed and high print quality, the most extensively used printers are electrophotographic printers. Laser printers and liquid-crystal printers are two examples of such printers, but in addition there is the LED printer, which uses a print head comprised of an array of LEDs. In the LED printer, printing comprises using image signals to drive the print head LED array and form an electrostatic latent image on a photosensitive drum, and the image is then developed and transferred to paper.
FIG. 1 is a plan view of a first example of a conventional LED print head that uses an LED array chip 11. In this arrangement, a number of LEDs are arrayed at a constant pitch P1 longitudinally (the direction indicated by an arrow 12) along an LED array chip 11, and emit light in accordance with image signals supplied via the respective electrodes. The electrodes are arranged along each side across the width of the chip 11 (the direction indicated by arrow 17) so that the electrodes 15 of odd-numbered LEDs 13 are at one side and the electrodes 16 of even-numbered LEDs 14 are at the other in a staggered formation. As a result, at each side, the electrode pitch P2 is twice the LED pitch P1.
FIG. 2 is a longitudinal cross-sectional view showing the LED print head in which the LED array chip 11 of FIG. 1 is used, and peripheral portions. Thus, in this arrangement the LED print head 21 is provided with the LED array chip 11 which is driven by a first external drive circuit 22 and a second external drive circuit 23. With reference to the drawing, the LED array chip 11 is shown with its longitudinal axis thereof perpendicular to the surface plane of the drawing sheet and its emission plane facing a self-focussing rod-lens array 29. Image signals 28 from an image signal supply circuit (not shown) are divided into odd-numbered and even-numbered image signals 26 and 27 for input to the respective drive circuits.
Wires 24 connect the first external drive circuit 22 to the electrodes 15 of the odd-numbered LEDs 13 (FIG. 1), whereby the LEDs 13 are driven in accordance with the image signals 26. Similarly, wires 25 connect the second external drive circuit 23 to the electrodes 16 of the even-numbered LEDs 14, whereby the LEDs 14 are driven in accordance with the image signals 27.
The image signals 28 that are input to the LED print head 21 thus configured are divided into odd-numbered image signals 26 and even-numbered image signals 27 for input to the first and second external drive circuits 22 and 23. These first and second external drive circuits 22 and 23 apply a current to electrodes 15 and 16 that corresponds to the input image signals 26 and 27. The current applied to the electrodes gives rise to light emission by the LEDs 13 and 14 and flows to ground via a common electrode (not shown) provided on the lower surface of the LED array chip 11.
The one-dimensional pattern of light emission thus produced by the LED array chip 11 is then focussed by the self-focussing rod-lens array 29 to form a print line image on a photosensitive drum 31. The self-focussing rod-lens array 29 is comprised of self-focussing rod-lenses for each of the component LEDs on the chip 11, the said rod-lenses being arrayed perpendicularly, with respect to the surface plane of the drawing sheet. An electrostatic latent image that corresponds to the pattern of light emission focussed by the self-focussing rod-lens array 29 is formed on the photosensitive drum 31, that rotates at a constant speed in the direction indicated by the arrow 32. The latent image is developed by a processing section (not shown) and then transferred to paper.
The width L of the LED array chip with odd-numbered and even-numbered electrodes thus arranged in a staggered configuration along the face of the chip, will therefore be: EQU L=2L1+L2 (1)
FIG. 3 is a plan view of a second example of a conventional LED print head that uses an LED array chip. With reference to the drawing, a number of LEDs 36 are arrayed at a constant pitch P1 longitudinally (the direction indicated by arrow 12) along an LED array chip 35, and emit light in accordance with image signals supplied via respective electrodes 37. These electrodes 37 are arranged along only one side of the chip, in the direction in which the LEDs are arrayed and, also, at a pitch P1. In this case the width L' of the LED array chip 35 will be: EQU L'=L1+L2 (2)
Printers with multilevel printing capability can employ area-based gradation or multiple density levels. In the majority of printers that employ an LED print head, area-based gradation is used in association with dithering. In such cases, the print head pixel density has to be several times higher than the printer resolution. For example, in the case of a printer that has a resolution of 600 dpi (dots per inch), the pixel density of the print head would have to be at least 1200 dpi. To realize this, the spacing pitch P1 of the print head LEDs has to be in the order of 21 micrometers. This means that the electrode pitch P2 in the case of the conventional art example shown in FIG. 1 would be 42 micrometers, and 21 micrometers in the case of the second conventional example of FIG. 3.
However, with the present state of wire bonding technology, to ensure that adjacent wires do not touch, a minimum spacing of about 40 micrometers between bonds is used. This means that present wire bonding techniques can be applied to the first conventional example but not to the second conventional example.
An electrode needs to be about 90 micrometers long, as the length of the diode contact section is about 10 micrometers, plus about 20 micrometers for the lead-in portion and 60 micrometers for the wire bonding portion. Based on equation (2), this means a chip width of 110 micrometers in the case of the second conventional example of FIG. 3, but in the case of the first conventional example the width would have to exceed 200 micrometers, giving the latter chip about twice the area of the former chip.
In the semiconductor device fabrication process where large numbers of devices are fabricated on a wafer of semiconductor material, the smaller the area per device, the better the fabrication efficiency and the lower the cost. As in the case of an LED array the length of the array is determined by the number and density of the pixels, any cost reductions to be derived from reductions in the size of the chip have to come from a decrease in the width of the chip. Hence, improving fabrication efficiency and reducing costs are difficult to achieve in the case of devices with a large chip width, such as that of the first conventional example of FIG. 1. Thus, reducing the area of conventional print head LED array chips has presented a major problem, given the limitations of current wire bonding technology.